This invention relates generally to an indexing mechanism for angularly displacing a workpiece with respect to a machine tool, and more particularly to a mechanism in which accurate indexing is effected by a pair of intercoupled face gears which are brought into perfect mesh by a jiggling action.
During certain manufacturing procedures, it is often necessary to carry out a series of sequential machining operations on various portions of a workpiece. To this end, a mechanism must be provided capable of accurately indexing the workpiece relative to the manufacturing tool.
In a typical indexing mechanism, the workpiece must be axially displaced as it is shifted from one angular position to another. Although the axial movement required for this purpose is quite small, the indexing mechanism must impress an axial restraining force on the workpiece during the machining operation. As a consequence, known forms of indexing mechanisms are somewhat inaccurate as well as being limited in their ability to support the weight of the workpiece. Also, such mechanisms impose limits on the cutting forces which can be applied to the workpiece.
The improved indexing mechanism disclosed in the Price et al. U.S. Pat. No. 3,795,155 (1974) possesses the important advantage of effecting indexing without any axial displacement of the workpiece, thereby obviating the drawbacks inherent in earlier forms of indexing mechanisms and affording a higher order of accuracy.
In the indexing mechanism described in the Price et al. patent, whose entire disclosure is incorporated herein by reference, the workpiece is attachable to the end of a spindle on which is mounted one gear of a coupled pair of face gears, the spindle being rotated by a reversible, electronically-controlled, air motor through a worm and worm gear. Rotation of the spindle causes both the workpiece and the first gear to rotate. The second gear of the pair is axially movable along the spindle but is not rotatable, this gear being shiftable with respect to the first gear.
After the spindle carrying the workpiece has been rotated by the air motor to cause the first gear to assume the same angular position, the second gear is axially shifted until it intermeshes with the first gear which is thereby radially locked in place.
According to the Price et al. patent, the indexing accuracy of the mechanism is a function only of the grinding accuracy of the opposed transverse surfaces of the coupling gears. We have found, however, that because of restrictions inherent in the drive system, the face gears in an arrangement of the type disclosed in the Price et al. patent will not seat exactly, and the indexing is therefore inaccurate.
A fundamental requirement to achieve indexing accuracy within the limits imposed by the precision of the face gears is that the face gears be pressed together axially with complete freedom. This freedom permits the gears to seek their own rotational position as determined solely by the precision-ground teeth. Face gears are currently being manufactured to provide indexing within plus or minus 2 seconds of arc. The full and exact achievement of this high degree of precision is a desideratum of the indexing mechanism.
Any and all rotational forces which inhibit the complete freedom of the face gears to seek a condition of exact mesh will detract from the precision of the indexing. There are three main sources of restriction which militate against the attainment of exact mesh. The first source is the restriction introduced by the drive and positioning mechanism. The second source is the friction of the main spindle bearings. The third source is the eccentric loading on the spindle by the workpiece and its fixture.
The nature of the second and third sources is self-explanatory. However, further analysis of the first source of restriction may be helpful toward an understanding of why in the Price et al. arrangement, a condition of exact mesh is not attainable.
The positioning servo system disclosed in the Price et al. patent which functions to bring the rotatable spindle to the required angle is accurate to within approximately 360 seconds of arc (i.e., 0.1 degree). This is sufficiently accurate to allow the face gears to begin their engagement into proper mesh, but it does leave a positional error of the shaft to be corrected by the face gears.
Since the face gear pair, once fully and exactly in mesh, is able to achieve an accuracy for each angular position within 2 seconds of arc or less, it then becomes necessary for the face gears to back drive the positioning servo mechanism in order to achieve a perfect mesh. For example, if the servo mechanism acts to position the worm and worm gear driving the spindle so that the rotating face gear comes to rest at, say, 100 seconds of arc displaced from the true position, and the cylinder pistons, then proceed to axially advance the non-rotating face gear into engagement with the rotating face gear, the rotating face gear and the shaft are then called upon to rotate 98 seconds to permit the face gears to achieve their correct alignment.
However, such rotation is not possible with the worm and worm gear drive disclosed by the Price et al. patent, for such gears are self-locking and cannot be reverse-driven by the shaft. With other drive arrangements, it is possible to avoid a self-locking action and thereby afford some degree of freedom to reverse-drive the mechanism, but whatever known type of drive system is employed, restrictions inevitably will be imposed to prevent the attainment of a state of perfect mesh.
In summary, regardless of how accurately one grinds the opposed face gear surfaces of the coupling pair, the indexing mechanism of the type disclosed in the Price et al. patent will nevertheless be lacking in indexing precision, in that the gears of the pair cannot be brought into perfect mesh.